How do Renegotiation and Triple Handshake Attacks work in TLS?

Both attacks refer to a man-in-the-middle (MITM) attacks, in which an adversary – acting as a MITM – tries to inject data before the client is properly authenticated. This data is then passed to the application together with the data provided by the client after authentication. Because the application cannot properly distinguish the data injected before authentication and the data provided afterwards, several attacks become feasible. The IETF originally crafted a renegotiation_info extension to mitigiate such attacks. In 2014, however, it was shown that this extension is not sufficient, and that a special form of renegotiation attack – a so-called triple handshake attack or 3SHAKE attack, in short – is still feasible, and hence the IETF had to craft another extension (called extended_master_secret ) to mitigate the attack. You may download a 7 page description of the attacks and the respective mitigation technologies here.